1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with compliant inserts mounted within holes in the upper tie plate of a BWR fuel assembly for supporting the upper end plug extensions of assembly fuel rods so as to prevent binding and axial loading of the fuel rods.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In nuclear reactors generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. Specifically, in a boiling water reactor (BWR) the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. A hollow water cross extends axially through the outer channel so as to provide an open inner channel for subcooled moderator flow through the fuel assembly and to divide the fuel assembly into four, separate, elongated compartments, each containing an identical mini-bundle of the fuel rods. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows through the water cross and along the fuel rods removing energy from their heated surfaces.
The fuel rods of each mini-bundle extend in laterally spaced apart relationship between an upper tie plate and a lower tie plate and connected together with the tie plates comprises a separate fuel rod subassembly within each of the compartments of the channel. A plurality of grids axially spaced along the fuel rods of each fuel rod subassembly maintain the fuel rods in their laterally spaced relationships. The water cross has approximately the same axial length as the fuel rod subassemblies, extending between the upper and lower tie plates thereof.
In each fuel rod subassembly of the BWR fuel assembly, the mini-bundle of fuel rods is composed of standard fuel rods and tie rods. Such use of standard and tie fuel rods is conventional, as can be seen in BWR fuel bundles illustrated in U.S. patents to Qurnell et al (U.S. Pat. No. 3,741,868) and Smith et al (U.S. Pat. No. 4,022,661). Ordinarily, the tie rods have extensions with nuts on the ends thereof which limit movement within holes in the upper tie plate, whereas the standard fuel rods have upper end plug extensions which are slidably received within holes in the plate. The upper tie plate is positioned axially by the tie rods, whereas the top ends of all the fuel rods, including the tie rods, are positioned and supported laterally by the upper tie plate via the pattern of holes defined therein.
Since thermal and irradiation growth rates may be different between the tie rod and the standard fuel rod, especially in fuel assembly designs where the fuel cladding is made of cold-worked Ziracaloy, the end plug extension on the standard fuel rod must slide freely in its receiving hole in the upper tie plate to accommodate relative growth in length between the tie and standard fuel rods. Otherwise, binding and an axial load in the fuel rod would result, which leads to bowing of the fuel rod.
Consequently, a need exists for a way to prevent binding of the upper end plug extension within the upper tie plate hole in order to avoid axial loading and resultant bowing of the fuel rod.